Optical reflectometric methods are widely employed for characterization of optical fiber links. Among these methods, the most commonly used approach is Optical Time-Domain Reflectometry (OTDR—also used to refer to the corresponding device), a diagnostic technique where light pulses are launched in an optical fiber link and the returning light, arising from backscattering and reflections along the fiber link, is detected and analyzed. Various “events” along the fiber link can be detected and characterized through a proper analysis of the returning light in the time domain.
Nowadays, most OTDRs on the market provide an automatic mode, where the instrument automatically chooses an appropriate pulse duration (also referred to as “pulsewidth”) and acquisition range (see, for example, the FTB-720 commercialized by EXFO or the OptiFiber™ Pro OTDR commercialized by Fluke Networks). In order to choose the appropriate settings for the final acquisition, the instrument launches one or many brief “investigation acquisitions”, which provide a quick overview of the link being tested. In general, the investigation acquisitions are hidden from the user, and only the final acquisition is made available.
For both manual settings and automatic settings, the final result is an OTDR trace performed with pulses having a common duration. In general, a given pulsewidth will be selected to allow characterization of the complete link. For example, a link having large loss requires testing with a long pulse. However, the use of a long pulse brings certain limitations in the ability to characterize short fiber sections, as well as closely spaced events.
An improvement to the single-pulsewidth approach has been developed, whereby the equipment makes use of multiple acquisitions performed with different pulsewidths. Such an approach is the basis of the Intellitrace Plus™ technology by Tektronix (http://www.tek.com, see also U.S. Pat. No. 5,155,439 (HOLMBO et al) and U.S. Pat. No. 5,528,356 (HARCOURT)) and of the Intelligent Optical Link Mapper (iOLM) technology by EXFO (see U.S. patent publication no. US 2013/0088718 A1 (PERRON et al), commonly owned by the Applicant and of which the specification is hereby incorporated by reference). In the above multiple-pulsewidth approaches, shorter pulses are used to characterize the most proximal portion, i.e. the portion closest to the OTDR, of the link under test with a better resolution, while longer pulses provide for more signal power to characterize portions of the optical link that are farther away but with a drawback on resolution. The above-described multiple-pulsewidth approaches therefore offer significant improvement to the traditional single-pulsewidth approach, as each event can be characterized by an “optimum” pulsewidth. In addition, in the iOLM approach by EXFO, each parameter of an event can be individually characterized using the “optimum” pulsewidth. For example, a first acquisition performed with one pulsewidth may be used to characterize the event location while a second acquisition performed with another pulsewidth may be used to characterize the insertion loss or the reflectance associated with the event. However, all the above-mentioned multiple-pulsewidth approaches are limited to single-ended measurements.
A problem with single-end OTDR measurements is the characterization of splice loss in cases where the link under test includes multiple fiber types. Differences in mode field diameter (MFD) corresponding to respective different concatenated optical fiber types in a link may lead to different degrees of backscattered light in response to OTDR test pulses. As a consequence, MFD mismatch between spliced fibers may cause an apparent “gain” or a drop in the backscattered light of OTDR measurements, which is not related to the real loss at the point of concatenation (e.g. at a fiber splice). For example, a fiber splice may appear as a gain in the backscattered light due to MFD mismatch between the spliced fibers. An OTDR measurement performed from the opposite end on the same fiber splice would conversely result in an overestimation of the splice loss. For this reason, single-end OTDR measurements do not allow for proper characterization of such events. The Telecommunications Industry Association (TIA) therefore recommends the use of bi-directional OTDR analysis to average the results of single-ended OTDR measurements and properly characterize multi-fiber links (test procedure EIA/TIA FOTP-61 “Measurement of Fiber or Cable Attenuation Using an OTDR”).
In accordance with bi-directional OTDR analysis of multi-fiber optical links, a single-end single-pulsewidth OTDR measurement is performed in each direction of the link under test, i.e. one measurement from each end. The information obtained from the two measurements is combined to characterize events identified along the link. In order to do this, the single-ended OTDR traces need to be matched in position along the link. The match is generally based on the position of multiple events appearing on the OTDR traces. Accordingly, each single-ended measurement is required to cover the entire link with an acceptable Signal-to-Noise Ratio (SNR) and consequently a proper pulsewidth of the test pulses should be selected to fulfill this requirement. The same pulsewidth is hence used to characterize the entire link. The spatial resolution of the OTDR measurement is therefore limited by this constraint and high-resolution bi-directional OTDR measurements are not possible.
There is therefore a need for bi-directional OTDRs allowing a better resolution at least for portions of the link under test close to the extremities thereof.